py/objtype: Avoid crash on calling members of uninitialized native type.

When subclassing a native type, calling native members in `__init__` before
`super().__init__()` has been called could cause a crash.  In this
situation, `self` in `mp_convert_member_lookup` is the
`native_base_init_wrapper_obj`.  The check added in this commit ensures
that an `AttributeError` is raised before this happens, which is consistent
with other failed lookups.

Also fix a typo in a related comment.

Signed-off-by: Laurens Valk <laurens@pybricks.com>

3 files changed, 49 insertions, 1 deletions

diff --git a/py/objtype.c b/py/objtype.c
index b6d600e943..f7a65a6ca5 100644
--- a/py/objtype.c
+++ b/py/objtype.c
@@ -82,7 +82,7 @@ static int instance_count_native_bases(const mp_obj_type_t *type, const mp_obj_t
     }
 }
 
-// This wrapper function is allows a subclass of a native type to call the
+// This wrapper function allows a subclass of a native type to call the
 // __init__() method (corresponding to type->make_new) of the native type.
 static mp_obj_t native_base_init_wrapper(size_t n_args, const mp_obj_t *args) {
     mp_obj_instance_t *self = MP_OBJ_TO_PTR(args[0]);
@@ -170,6 +170,12 @@ static void mp_obj_class_lookup(struct class_lookup_data *lookup, const mp_obj_t
                     if (obj != NULL && mp_obj_is_native_type(type) && type != &mp_type_object /* object is not a real type */) {
                         // If we're dealing with native base class, then it applies to native sub-object
                         obj_obj = obj->subobj[0];
+                        #if MICROPY_BUILTIN_METHOD_CHECK_SELF_ARG
+                        if (obj_obj == MP_OBJ_FROM_PTR(&native_base_init_wrapper_obj)) {
+                            // But we shouldn't attempt lookups on object that is not yet instantiated.
+                            mp_raise_msg(&mp_type_AttributeError, MP_ERROR_TEXT("call super().__init__() first"));
+                        }
+                        #endif // MICROPY_BUILTIN_METHOD_CHECK_SELF_ARG
                     } else {
                         obj_obj = MP_OBJ_FROM_PTR(obj);
                     }
diff --git a/tests/misc/cexample_subclass.py b/tests/misc/cexample_subclass.py
new file mode 100644
index 0000000000..9f52a2c737
--- /dev/null
+++ b/tests/misc/cexample_subclass.py
@@ -0,0 +1,37 @@
+# test subclassing custom native class
+
+try:
+    from cexample import AdvancedTimer
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+
+class SubTimer(AdvancedTimer):
+    def __init__(self):
+        # At this point, self does not yet represent a AdvancedTimer instance.
+        print(self)
+
+        # So lookups via type.attr handler will fail.
+        try:
+            self.seconds
+        except AttributeError:
+            print("AttributeError")
+
+        # Also applies to builtin methods.
+        try:
+            self.time()
+        except AttributeError:
+            print("AttributeError")
+
+        # Initialize base class.
+        super().__init__(self)
+
+        # Now you can access methods and attributes normally.
+        self.time()
+        print(self.seconds)
+        self.seconds = 123
+        print(self.seconds)
+
+
+watch = SubTimer()
diff --git a/tests/misc/cexample_subclass.py.exp b/tests/misc/cexample_subclass.py.exp
new file mode 100644
index 0000000000..a035649e47
--- /dev/null
+++ b/tests/misc/cexample_subclass.py.exp
@@ -0,0 +1,5 @@
+<function>
+AttributeError
+AttributeError
+0
+123